Want to suss out the existence of a shy mammal in a tropical jungle? Just check a bloodsucking leech.

Scientists estimate that about a quarter of the world’s mammal species are under threat of extinction. And researchers want to know where these mammals live. The problem? Many are tough to find. Especially in dense tropical rainforests.

So researchers in Denmark got the idea of testing a proxy: leeches, which feed off those elusive mammals. First they figured out that a mammal’s DNA could still be found in a leech up to four months after it got slurped. Then the researchers headed to a Vietnamese jungle.

They collected 25 leeches. Which provided evidence for six mammal species, some rare. One, an Annamite striped rabbit, had been described there in 1996 but not been found since. The research was published in the journal Current Biology. [Ida Bærholm Schnell et al., "Screening mammal biodiversity using DNA from leeches"]

The leech technique is easy to teach to local residents, and could yield important information about biodiversity in a region. Plus, unlike reclusive mammals, leeches leap at the opportunity to approach humans. But instead of getting a meal, the leeches will feed us their collection of DNA data.